Bitlocker TPM+PIN - Safety only when TPM is safe?

The SHA-256/160-bit description matches Microsoft's BitLocker FAQ, but the PIN is authorization data for sealing and unsealing the volume master key, rather than the disk-encryption key itself.

A TPM vulnerability needs a specific threat model: bypassing authorization, extracting sealed material and intercepting an already released key are different attacks. Microsoft's countermeasures still recommend TPM+PIN against attackers with extended physical access; they do not describe it as protection against every possible hardware compromise.

Your proposed extra derivation would require cryptographic design and implementation review. Both hash portions still originate from the same PIN, so they are not independent user secrets. The published BitLocker guidance does not establish Microsoft's rationale for omitting your particular construction.

For a supported additional factor, evaluate the combined TPM, startup-key and PIN protector. Verify that you configure the combined protector, rather than adding a separate alternative unlock method, and retain a protected recovery key.